The adherence of a film layer to a substrate such as a panel, a semiconductor wafer or other workpiece during a deposition process depends to a great extent on the cleanliness of the workpiece surface prior to deposition. Many semiconductor processes such as physical vapor deposition (PVD) rely on ultra high vacuum (UHV) chambers for the most favorable processing conditions and performance. An ultra high vacuum delays the growth of contaminating films on the wafer surface prior to deposition, because materials, such as oxygen, which react with, or deposit on, the substrate to form contamination films, are present in very small quantities in an UHV. In general, a UHV chamber used for PVD should operate at pressures of lower than about 10.sup.-8 torr. A cryogenic pump (cryo-pump) may be utilized in order to achieve such low pressures. A PVD system may employ more than one vacuum pump, for example, a roughing pump for achieving about 10.sup.-2 torr and a cryo-pump for achieving lower pressures. One such cryo-pump useful for achieving high vacuum is manufactured by CTI Cryogenics.
Between the chamber and the cryo-pump is typically provided a gate valve having a passage coupling the chamber to the cryo-pump. The passage in the gate valve may be opened and closed in order to open or close off the cryo-pump from the chamber. Such a gate valve is manufactured, for example, by VAT Incorporated, of Woburn, Mass.
When a vacuum chamber is first pumped to vacuum, such as after it has been opened or cleaned, materials which have adhered on or in the vacuum chamber surfaces can detach therefrom as vacuum is achieved. In many cases, these materials, one being water vapor, will generally continuously leak into the chamber and thereby make it difficult or impossible for the chamber to achieve UHV. Therefore, a bake out step may also be utilized in which the reaction chamber is heated and contaminates are desorbed or evaporated from the chamber surfaces by the heating and then pumped out of the chamber.
Contaminants may accumulate due to a variety of causes, including (1) deposition processes; (2) replacement of the process kit (target and shield) or other consumables; (3) parts wearing out; and (4) anytime the chamber is vented to the atmosphere and contaminants enter the system. It is desirable to prevent the accumulation of such contaminants in order to reduce pump down time and to increase the yield during processing.
Chamber pump down and initial bake out to remove contaminants prior to semiconductor wafer processing are time consuming steps. Prior to commercial use, PVD systems are often qualified to ensure that the system can achieve the necessary vacuum conditions for wafer processing. Qualification is typically accomplished by baking out and pumping down the chamber to establish that proper vacuum conditions can be achieved. Furthermore, any time the chamber is vented and exposed to atmosphere, the chamber may be baked out. The chamber bake out time and the pump down time are affected by various factors, including the pumping speed and efficiency of the cryo-pump, the surface area available for gas flow into the port of the cryo-pump, the quantity of contaminants in the chamber and the release rate of the contaminants from the interior surfaces of the chamber.
The chamber may have internal heating elements for use during the bake out step. In addition to internal heaters for chambers, there also exist flexible external heating jackets that may be wrapped around external components such as inlet pipes to the chamber in order to bake out and prevent accumulation of contaminants. Such flexible heating jackets typically have a flexible heating pad that is wrapped around the outside surface of a component. Flexible heating jackets are manufactured by HPS Division of MKS Instruments, Inc.
Flexible heating jackets are generally designed for products with rounded surfaces or surfaces that are smooth, so that the entire surface of the device can be covered by the jacket. Thus, heating jackets are typically used for wrapping around relatively small parts such as rounded pipes and fittings. Large components may be difficult to adequately heat using a heating jacket, due to the large volume that must be heated and the heat loss through the jacket itself. Furthermore, flexible heating jackets may present a hazard to the user due to elevated temperatures on the jacket surface.
In addition, flexible heating jackets have a relatively short expected lifetime and can wear out during repeated use. Components for use on a PVD system should be designed to minimize the chance that particles will flake off and contaminate the system. As a flexible heating jacket wears out, the risk of particle flake-off and system contamination will increase.
Despite the use of these known internal and external heaters, bake out and pump down typically remain very time consuming. As a result, processing is delayed and the overall productivity of the processing machines is reduced.